1. Field of the Invention
The present invention relates to a system for providing voice sessions, in particular, to a method for providing voice sessions with a good voice quality in voice sessions between three or more parties.
2. Description of the Related Art
A new communication service called PoC (Push-to-Talk over Cellular) has been attracting attention. The PoC is also referred to as PoC/W (Push-to-Talk over Cellular/Wireless), and is a transceiver-type voice session service that allows two-way conversation Unlike conventional IP (Internet Protocol) phone services, such as a VoIP (Voice over Internet Protocol) service, the PoC has features that the PoC allows a voice packet to be simultaneously transmitted to a plurality of recipients (PoC clients), and allows the recipients to receive voice messages without performing any actions for answering. Due to these features, the PoC service has been attracting interests in numerous fields as an easy communication tool among a group. Users push a talk button provided on PoC clients, thereby acquiring permission to talk (a talk right) from a PoC server, and talking. Only one client in the group can acquire the talk right from the PoC server at a time. PoC clients perform communication with each other while repeating acquisition and release of the talk right.
Call terminals that receive a voice packet, which is a unit of transmission of voice information, convert voice data contained in the voice packet into a voice signal, and output the reproduced voice sound from a loudspeaker. In case that the following voice packet does not arrive even after finishing the output of the reproduced voice sound, a pause is caused after the reproduced voice sound. As jitter in packet transmission time is caused due to a condition of the network, such pauses frequently appear if voice sound is immediately reproduced from the received voice packets. That is, such quality degradation occurs that the reproduced voice sound frequently breaks. To avoid this, reproduction of voice sound is not performed immediately after receiving the voice packets. Instead, the voice data is accumulated in a buffer, and after some delay, reproduction of voice sound is started. By this, although breaks in voice sound become less, the time to the start of reproduction is delayed. Since the jitter in the packet transmission time is great in a mobile network, it is necessary to provide a large-capacity buffer and to delay the start of reproduction significantly in order to perform high-quality voice-packet transmission. Large time delay to the start of reproduction cannot be set in IP phones, which requires short voice delay. Thus, it is difficult to introduce IP phones into a mobile network. However, the PoC can permit larger voice delay compared with the IP phones because full duplex voice-packet transmission is not performed in the PoC, and can set a large time delay to the start of reproduction. Accordingly, introduction of the PoC is easy even on a mobile network. Voice-packet transmission in the PoC is performed in accordance with a specification defined in RFC (Request for Comments) 1889 of IETF (Internet Engineering Task Force) (H. Schulzrinne and three others, “RTP: A Transport Protocol for Real-Time Applications”, January 1996, Internet Engineering Task Force).